234 NEW YORK STATE MUSEUM 
early differentiating process had to deal with a long median fork, 
with a cross vein at each elbow of it. But the median cross vein 
standing in its midst and binding its arms together beyond the cord 
and opposite the fork, preventing their spreading, cle rly corrects in 
some measure the obvious weakness of this arrange: went. 
In our diagram [fig. 13] the cell is represented between the cross 
veins and adjoining forks, like a ring slung in a cord. It required 
the median cross vein to complete the ring. This is the reason why 
that cross vein is far more persistent than any other outside the 
cord. There can be no doubt of this, for that cross vein disappears 
only when the cord is shifted to the proximal end of cell 1st M?, 
and it is thereby put out of commission. The testimony of the 
figures in the plates given herewith is unmistakable as to this. Very 
rarely, as in Conosia [pl. 21, fig. 5], there is a shift of the cord 
distally, which brings the median cross vein more directly! into 
the line of stress: in such a case it would never be lost. 
The forward shift of veins Cut and M?. The tendency of vein 
Cu’ to be deflected forward at its base and strongly joined to media 
has been noted in the preceding pages. The accompanying diagram 
[fig. 15] illustrates successive stages in the progress of that tend- 
Fig. 15 
ency. All these are abundantly illustrated in the plates accompany- 
ing this paper, and one figure, that of Diotrepha [pl. 29, fig. 6], illus- 
trates a far more extreme case. By the means here shown the tip of 
the vein Cu’ comes to be attached directly upon the base of media and 
‘in direct line therewith, and it has been usually interpreted as a 
branch of the same. Ordinarily, this union is a strong one, and the 
deflected portion of Cut is one of the stoutest veins of the wing, 
as it is in many other Diptera. But among the crane flies are found 
*It may be noted in passing that in the Lepidoptera an outward shift of 
stresses, somewhat like that shown in Conosia, has brought the median 
cross vein permanently into the cord, and the other proximal part of the 
first. median fork has atrophied, leaving three cells, the so called first and 
second basal cells and the discal cell of the dipterous wing, to constitute 
together, when their intervening boundaries are atrophied, the “discal cell” 
of the Lepidoptera. | 
